Initiation of liquefaction

To understand how soil liquefaction is initiated, some basic soil mechanics concepts
are important. They are briefly described below. For a more thorough explanation, see
the references.

Critical Void Ratio
In 1936, Dr. Arthur Casagrande performed a series of
drained straincontrolled triaxial tests and discovered that initially loose and dense
specimens at the same confining pressure approached the same density when sheared to
large strains. The void ratio corresponding to this density was called the critical
void ratio (e_{c}).


Behavior of dense and loose soils in
monotonic strain controlled triaxial tests (after
Kramer, 1996).


Performing tests at various effective confining pressures, Casagrande found that the
critical void ratio varied with effective confining pressure. Plotting these on
a graph produced a curve which is referred to as the critical void ratio (CVR) line.
The CVR line constituted the boundary between dilative and contractive behavior in
drained triaxial compression. A soil in a state that plots above the CVR line
exhibits contractive behavior and vice versa (see figure below).



CVRline for arithmetic and logarithmic confining
pressure.


Steady State of Deformation
In the mid1960s, Gonzalo Castro, a student of
Casagrande, performed an important series of undrained, stresscontrolled triaxial tests.
Castro observed three different types of stressstrain behavior depending upon the soil
state. Dense specimens initially contracted but then dilated with increasing effective
confining pressure and shear stress. Very loose samples collapsed at a small shear strain
level and failed rapidly with large strains. Castro called this behavior
"liquefaction"  it is also commonly referred to as flow liquefaction. Medium
dense soils initially showed the same behavior as the loose samples but, after initially
exhibiting contractive behavior, the soil "transformed" and began exhibiting
dilative behavior. Castro referred to this type of behavior as "limited
liquefaction".




Static triaxial test stress paths for
three specimens of different densities.


Castro plotted the relationship (see figure below) between effective confining pressure
and void ratio at large strains for these undrained, stresscontrolled tests. Castro
referred to the curved produced by this plot, which is similar to the CVR line for the
drained strain controlled tests performed by Casagrande, as the Steady State Line (SSL).
The difference between the CVR and SSL was attributed to the existence of what
Casagrande called a "flow structure", in which the grains orient themselves so
the least amount of energy is lost by frictional resistance during flow.



Left: 3D steady state line. Right: 2D
Projection of SSL plotted on graph of void ratio versus the logarithm of confining
pressure or steady state strength.


As seen above, the SSL is actually a 3dimensional curve in e
s't space. Using the 2D
projection on the es' plane (see figure above), one can
determine if a soil is susceptible to flow liquefaction. Soils in an initial state
that plots below the SSL are not susceptible to flow liquefaction whereas soils
plotting above the SSL are susceptible to flow liquefaction  if (and only if) the
static shear stress exceeds the residual strength of the soil. Cyclic mobility, another
liquefactionrelated phenomenon, can occur in dense as well as loose soils.


Figure showing zones of flow liquefaction
and cyclic mobility susceptibility.


Flow Liquefaction
On the left below is a plot of stress paths for
five undrained shear tests. Three test specimens (C, D, and E) were subjected to
loads greater than their residual strengths, and experienced flow liquefaction. A
straight line (shown in red in the figure) drawn through the points where flow
liquefaction was initiated projects back through the origin. This line is called the
Flow Liquefaction Surface (FLS). Since flow liquefaction cannot take place if the static shear
stress is lower than the steady state strength, the FLS is truncated by a horizontal
line through the steady state point (see right figure below).The steady state strength
is the strength a soil has when undergoing a steady state of deformation, i.e. continuous flow
under constant shear stress and constant effective confining pressure at constant volume and constant
velocity. Flow liquefaction will be initiated if the stress path crosses the FLS during undrained
shear regardless of whether the loading is cyclic or monotonic loading (
Vaid and Chern, 1983).


Graphical explanation of Flow
Liquefaction Surface.


The stress paths for monotonic and cyclic loading can be seen below. The flow
liquefaction process can be described in two stages. First, the excess pore pressure
that develops at low strains moves the effective stress path to the FLS, at which point
the soil becomes unstable. When the soil reaches this point of instability under
undrained conditions, its shear strength drops to the residual strength. As a result
the static shear stresses drive the large strains that develop as the soil
"collapses". A great amount of strainsoftening takes place when the stress
path moves toward the steady state point.

Flow Failure induced by cyclic and monotonic loading.

Cyclic Mobility

Cyclic mobility can occur even when the static shear stress
is lower than the steady state (or residual) shear strength. The geotechnical engineering
profession's understanding of cyclic mobility has advanced greatly within the past 10 years
or so.
A key to this understanding came about with identification of the phase transformation line.
Medium dense to dense sands subjected to monotonic loading will initially exhibit contractive
behavior, but then exhibit dilative behavior as they strain toward the steady state. A plot
of the stress path points at which the transformation from contractive to dilative behavior takes
place reveals a phase transformation line (PTL) that appears to project back through the origin
(Ishihara, 1985).

A p'q plot of the phase transformation line

In the contractive region, an undrained stress path will tend to move to the left
as the tendency for contraction causes pore pressure to increase and p' to decrease.
As the stress path approaches the PTL, the tendency for contraction reduces and the
stress path becomes more vertical. When the stress path reaches the PTL, there is no
tendency for contraction or dilation, hence p' is constant and the stress path is vertical.
After the stress path crosses the PTL, the tendency for dilation causes the pore pressure
to decrease and p' to increase, and the stress path moves to the right.

A stress path example.

Note that, because the stiffness of the soil depends on p', the stiffness decreases
(while the stress path is below the PTL) but then increases (when the stress path
moves above the PTL). This change in stiffness produces the "limited liquefaction"
behavior originally noted by Castro.
Under cyclic loading conditions, the behavior
becomes even more complex. Remembering that the failure envelope and PTL exist for
negative shear stresses as well as positive, it is easy to see that a cyclically loaded
soil can undergo the contraction/dilation transformation in two different directions.
The stressstrain and stress path plots for a harmonically loaded element of soil will
therefore show softening behavior in the early stages of loading (before the stress path
has reached the PTL) but then show cyclic softening and hardening as the stress path moves
from one side of the PTL to the other. The result of the phase transformation behavior is
reflected in the development of "bananashaped" stressstrain loops.

Cycmob3.gif

Evaluation of Liquefaction Potential

Evaluation of the potential for liquefaction to occur is accomplished by
comparing equivalent measures of earthquake loading and liquefaction resistance.
The most common approach to characterization of earthquake loading is through the
use of cyclic shear stresses. By normalizing the cyclic shear stress amplitude
by the initial effective vertical stress, a cyclic stress ratio (CSR) can represent
the level of loading induced at different depths in a soil profile by an earthquake.
There are different procedures for evaluating the cyclic shear stresses  site
response analyses may be performed or a "simplified" approach may be used to estimate
CSR as a function of peak ground surface acceleration amplitude.

CSR versus N or q_{c}

Liquefaction resistance is most commonly characterized on the basis of observed field
performance. Detailed investigation of actual earthquake case histories has allowed
determination of the combinations of insitu properties (usually SPT or CPT resistance)
and CSR for each case history. By plotting the CSR(N1)60 (or CSRqc) pairs for cases
in which liquefaction was and was not been observed, a curve that bounds the conditions
at which liquefaction has historically been observed can be drawn. This curve, when
interpreted as the maximum CSR for which liquefaction of a soil with a given penetration
resistance can resist liquefaction, can be thought of as a curve of cyclic resistance
ratio (CRR). Then, the potential for liquefaction can be evaluated by comparing the
earthquake loading (CSR) with the liquefaction resistance (CRR)  this is usually expressed
as a factor of safety against liquefaction,

FS = CRR / CSR 
A factor of safety greater than one indicates that the liquefaction resistance exceeds the
earthquake loading, and therefore that liquefaction would not be expected.
